This invention relates generally to positive displacement piston pumps, and more particularly, is directed to a positive displacement piston pump that prevents the entrapment of air during operation.
Positive displacement piston pumps, to which the present invention is directed, are well known, for example, from U.S. Pat. Nos. 3,168,872; 3,257,953; and 4,008,003. Such pumps include a cylinder having an inlet port and a diametrically opposite outlet port. A piston is rotatably and reciprocably driven in the cylinder and includes a recessed section at the free end thereof, which functions as a duct between the inlet port and the outlet port. During rotation of the piston, the recessed section is alternately in fluid communication with the inlet port and the outlet port, whereby fluid is pumped from the inlet port to the outlet port. During rotation, the piston also reciprocates within the cylinder between a retracted position and an extended position, the latter corresponding to the end of the pressure stroke.
The piston is secured to a drive shaft of a motor by means of a pivotal coupling. Specifically, a yoke is keyed to the drive shaft of the motor and includes a socket accessible through a bore of the yoke. A transverse arm is secured to the driven end of the piston and has a ball formed at the free end thereof which mates with the socket to form a universal ball and socket joint. In this regard, the piston and cylinder can be pivoted with respect to the axis or center line of the drive shaft of the motor.
The angle between the piston and the drive shaft determines the pump stroke and the direction of pumping. When the axis of the piston is coincident with the center line of the drive shaft, the piston does not reciprocate in the cylinder during rotation of the drive shaft. Under such circumstances, no pumping action takes place. When the piston is pivoted with respect to the drive shaft in a first direction, reciprocation occurs during rotation. The amount of reciprocation depends on the angle between the piston and drive shaft. As the angle is increased, the piston stroke is increased and the flow rate is increased between the inlet port and the outlet port. When the piston is pivoted with respect to the drive shaft in the opposite direction, the flow is reversed, so that the former inlet port and outlet port become the outlet port and inlet port, respectively. Again, the amount of reciprocation depends on the angle between the piston and drive shaft.
However, a problem occurs with use of such pumps, particularly when used for the precision metering of fluids requiring low flow rates, for example, on the order of a few milliliters per minute or less. Specifically, gases, such as air, hydrogen, carbon dioxide and the like which are carried in the fluid, are often released in the cylinder as a result of agitation of the fluid during the puxping operation or as a result of pressure and temperature changes. For example, some fluids respond to agitation and/or pressure and temperature changes by chemically separating into liquid and gas fractions, while other fluids simply vaporize, physically changing from liquid to gaseous form. The problem that results is that the gases form bubbles which become trapped in the pumping head of the cylinder, thereby spoiling the metering precision of the pump, and in some situations, blocking flow completely. Generally, the gas bubbles become trapped between the recessed section of the piston and the inner wall of the cylinder.
Specifically, when the piston is pivoted with respect to the drive shaft to its maximum extent, that is, when the pump is operating at maximum pump stroke, the piston reciprocates over a maximum distance between its retracted position and extended position such that the free end of the piston is in close proximity to the end wall of the cylinder in the extended position, that is, at the end of its pump stroke. In this position, the top or proximal end of the recessed section is at or below the outlet port, and is positioned in the working chamber of the cylinder which is bounded by the outlet port and the end wall. Any bubbles that are formed thereby exit through the outlet port.
However, when the pump is not operating at full capacity, that is, when the piston is pivoted to less than its maximum extent, the piston is caused to reciprocate over a lesser distance between its retracted position and extended position. As a result, the top of the recessed section remains above the outlet port at all times during reciprocation of the piston. Gas bubbles formed between the recessed section and the inner wall of the cylinder thereby remain during the pumping operation, adversely affecting the same. It will be appreciated that the smaller the piston stroke, the more gas that will be trapped by the recessed section, thereby increasing the ratio of volume of entrapped gas to pump displacement. In other words, the pump becomes gas sensitive.